Language/Generics/typedef_A09_t04.dart

// Copyright (c) 2019, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

/// @assertion The requirement that satisfaction of the bounds on the formal type
/// parameters of a generic type alias [D] must imply satisfaction of all bounds
/// pertaining to every type that occurs in the body of [D] limits the expressive
/// power of generic type aliases. However, it would require the constraints on
/// formal type parameters to be expressed in a much more powerful language if we
/// were to allow a significantly larger set of types to be expressed using a
/// generic type alias.
/// For example, consider the following code:
///     class A<X extends void Function(num)> {}
///     typedef F<Y> = A<void Function(Y)> Function(); // compile-time error
/// There is no way to specify a bound on [Y] in the declaration of [F] which
/// will ensure that all bounds on the right hand side are respected. This is
/// because the actual requirement is that [Y] must be a supertype of [num], but
/// Dart does not support lower bounds for type parameters. The type [A<void
/// Function(U)> Function()] can still be specified explicitly for every [U]
/// which satisfies the bounds declared by [A]. So the types can be expressed,
/// they just cannot be abbreviated using a generic type alias.
/// @description Checks that example from the Spec throws compile error as
/// expected.
/// @Issue 42436
/// @author iarkh@unipro.ru

class A<X extends void Function(num)> {}
typedef A<Y> Testme<Y>();
//        ^
// [analyzer] unspecified
// [cfe] unspecified

main() {
}